1. Field of the Invention
The present invention relates to thermal indicating compositions, and more specifically to such compositions containing one or more polydiacetylenes (PDAs), plus an oxide, such as ZnO, alloyed with a transition metal oxide, such as ZrO2 and/or TiO2; wherein, such composition will respond repeatedly to thermal stimuli at a desired “trigger” temperature, or wherein, such compositions can be used to establish the cumulative time of exposure to a given thermal stimuli level based upon the resulting chromaticity thereof.
2. Description of the Related Art
Materials that change color in response to external stimuli are known as “chromic materials”. Such chromic materials may radiate, lose color, or change properties induced by external stimuli. Different stimuli result in different responses in the material being affected. “Chromic” is a suffix that means color, so chromic materials are named based on the stimuli energy affecting them, for example: (1) photochromic—light, or (2) thermochromic—heat, or (3) piezorochromic—pressure, or (4) solvatechromic—liquid, or (5) electrochromic—electricity/voltage conflicts.
An example of a commercial utilization of thermochromic paint was the paint introduced by Mattel® Toy Corp. in the 1980's applied to their Hot Wheels Color Racers® and Color FX™ cars. These cars were painted with temperature sensitive paint which changed colors when placed in icy cold or warm tap water. These convention types of chromic paints change color upon exposure to certain temperatures, then change back to their initial state (and exhibit their original color) once the stimuli (i.e., temperature) is removed (in this example, the temperature of the paint returns to room temperature), i.e. reversibly.
Polydiacetylenes (“PDAs”) are a series of conjugated polymers which can undergo thermochromic transitions when exposed to temperature stimuli. This chromic change, as illustrated in FIG. 1, is caused by decrease of the conjugation length of the polymeric backbone due to strain induced by breaking of hydrogen bonds on the side groups. Therefore, the thermochromic transition temperature is a function of the side groups on the polymeric backbone structures. By changing the side groups, repeatable response to set stimuli are possible, allowing these materials to function as sensors. There are several applications of such PDAs, particularly in the form of coatings or films, as chromic sensors for temperature, chemical, and stress. These polymers are tailored to create inks, paints, and coatings that will, for example, with an irreversible color change indicate that an object has been exposed to a high temperature so as to impact its functionality.
The monomers making up the PDAs are typically colorless and become increasingly colored with polymerization. Color in PDAs occurs as a result of π to π* electronic transitions associated with the C≡C—C≡C diacetylene backbone. Reversible changes in the color of the polymer occur due to molecular conformational changes resulting from modifications of the side chain packing, ordering and orientation. This also means that these PDAs will undergo phase changes in two stable states, the low temperature blue state and the high temperature red state. Complete thermochromic reversibility from the red to the blue phase is known to take place in PDAs where sufficiently strong hydrogen bonding interactions exist and are recovered on cooling from the high temperature red state. Recovery of the hydrogen bonding interactions can also be induced by the addition of specific organic molecules. It was therefore surprising that the addition of the inorganic compound ZnO to the PDAs induced chromatic reversibility and a large upshift of the chromatic transition temperature. By contrast, it has been published that the addition of TiO2 and ZrO2 did not affect the chromatic transition parameters.
Cost effective, commercially available, PDA monomers provide irreversible blue to red transitions at temperatures ranging from about 145 to about 172 degrees F.—some examples being:
10,12 pentacosadiynoic acid (PCDA) at about 145° F.,
10,12 tricosadiynoic acid (TCDA) at about 165° F., and
10,12 docosadiynedioic acid (Bis-1) at about 172° F.
However, there are applications where it is necessary to significantly increase these temperature ranges. An important example is what occurred during Desert Storm, where the U.S. forces faced operational temperatures inside munitions' containers exceeded 190° F., i.e. far in excess of the design limits of about 145 to about 165° F. for such munitions.
It is further critical to understand how long a particular munition has been subjected to elevated temperatures over the design limitation thereof—as there are thermal stabilizer(s) provided in military munitions which are depleted over time by such exposure—and as, when such stabilizer(s) is depleted the munition can go critical. Current U.S. Army requirements would have such a means identify/remember/indicate when over any 3 days, an aggregate of 2 hours exposure above 160° F. has occurred. Currently there is no simple, economical means to know that such a period what temperature exposure has occurred with respect to fielded munitions.
Finally, as munitions are stored for extended periods, often greater than 20 years, as well as, being subjected to prolonged and repeated periods of transportation, the use of powered devices and electronics is impractical for temperature monitoring. Further, as stated above, current PDAs are limited with respect to indicating high trigger temperatures and are not known to be capable of identifying a period of exposure to any given temperature level. Therefore, having a non-powered, cost effective, reliable, and easily readable means to measure and indicate exposure to higher temperature levels and to the duration thereof, of a munition is critical to understand if that munition has been compromised and may represent a deadly hazard.